One interesting concept is the re - imagining of characters. For example, in an alternate universe, a classic hero could be a villain. Another concept is the different power structures. Maybe magic is used in a completely new way, like being a currency instead of a weapon. Also, the alternate social hierarchies can be fascinating. A race that was once oppressed could be the ruling class.
One interesting concept is the idea of meeting alternate versions of oneself. It can lead to self - discovery and exploring different personalities. Another is the discovery of unique cultures and societies in the alternate universes. For example, a world where magic is the norm instead of science. And then there's the concept of changing the course of events in an alternate universe, which can have far - reaching consequences.
One interesting concept could be a Naruto AU where the main characters have different elemental affinities. For example, Naruto might be a water - type user instead of wind. Another concept could be a world where the Akatsuki are the heroes. They could be fighting against an oppressive regime that the Hidden Villages support. Also, a fanfic could explore a Naruto AU where the tailed beasts are free and coexist peacefully with humans from the start.
Well, in Star Wars alternate universe fanfic, a cool concept is the existence of Force - sensitive species that were never mentioned in the original canon. Maybe there's a species that can manipulate the Force in ways that are completely different from Jedi or Sith, like being able to use it to communicate with all forms of life on a planet simultaneously. Another interesting thing could be a story where the Galactic Empire never fell, but instead evolved into a more benevolent regime over time due to some internal reforms. And then there are fanfics that explore what would happen if a character like Han Solo was a Jedi from the start, which would change a whole lot of the storylines we know.
One interesting concept could be alternate versions of the main characters' relationships. For example, in the regular universe, Kirk and Spock might have a certain dynamic, but in the alternate universe fanfic, they could be rivals or have a completely different friendship. Another concept might be different technological developments. Maybe in this alternate universe, the Federation has developed a new type of warp drive that allows for instantaneous travel. Also, there could be changes in the political landscape of the Star Trek universe. Instead of the Federation being a dominant force for peace, it could be at war with other species more often.
One interesting concept could be the different versions of the characters in the alternate universes. For example, a Ruby in a steampunk - themed alternate universe might have a different weapon design and fighting style. Another concept could be how the relationships between the characters change. Maybe in one alternate universe, Weiss and Blake are bitter enemies instead of friends. Also, the alternate universes could have different rules for Dust, which would lead to unique battles and power - ups.
One interesting concept could be alternate versions of the main characters. For example, Mulder might have a completely different backstory or personality traits. Maybe in this alternate universe, he's not as obsessed with the paranormal and has a more normal life as a regular FBI agent.
One interesting concept could be a world where the gods have lost their powers and the demigods have to find a new way to survive. Another might be an alternate universe where the camps, like Camp Half - Blood and Camp Jupiter, are merged from the start. And perhaps a universe where the monsters are the ones in control and the demigods are the hunted.
One interesting concept could be a world where Voldemort won the war. In this alternate universe, Hogwarts might be a very different place, with dark magic being taught openly. Another concept could be a universe where the houses are not divided as they are in the original. Maybe students are grouped based on different criteria like magical talents other than just bravery, intelligence, etc. For example, there could be a group for those who are good at magical creatures handling.
Some fanfictions in the Avengers alternate universe explore the idea of the Avengers being part of a post - apocalyptic world. In this setting, they might have to fight against new and more dangerous threats that emerged from the ruins. Maybe there are mutated creatures or a new breed of supervillains. Also, an alternate universe could feature a younger version of the Avengers, like teen versions, and their journey of growing into heroes while dealing with the typical teenage problems along with the super - hero stuff.
One interesting concept could be if the characters had different careers. For example, instead of being in a glee club, they could be professional dancers in a big city dance troupe. Another concept might be a world where the glee club is a secret society with special powers related to music.